1. Field of the Invention
The present invention relates to an optical waveguide and a method for fabricating thereof, which waveguide preferably applies to an optical integrated circuit (an optic-electronic integrated circuit) on which various optical devices and elements are closely arranged used for wavelength division multiplexed (WDM) optical transmission system.
2. Description of the Related Art
With the significant increase in data traffic in accordance with a world-wide spread of Internet, people value a WDM optical transmission system more highly than ever for the purpose of realizing a photonic network operable to catch up with the significant increase in data traffic.
In order to realize such a WDM optical transmission system, not only various optical elements and devices but also one or more optical active devices (e.g., LD (laser diode), PD (photo diode)) and electronic devices or elements can be integrated by using PLC (Planer Lightwave Circuit) technology. Further, a PLC is in the form of a multi layer having two or more waveguides so as to be more integrated.
An optical device formed by an optical waveguide is exemplified by an optical directional coupler (divider/coupler) thereby serving as a waveguide-type optical directional coupler (divider/coupler).
A multi-layer PLC includes a first waveguide 110 formed by a first cladding 101, a first core 102 and a second cladding 103, and a second waveguide 111 formed by second cladding 103, a second core 104 and a third cladding 105, as shown in FIGS. 11A through 11C. First waveguide 110 and second waveguide 111 are on a Si substrate 100. First core 102 and second core 104, through which signal light is guided respectively as first and second waveguides 110 and 111, are arranged so as to come partially closer to each other (the closer part is called an optical coupling portion) so that signal light propagating respectively through first core 102 and second core 104 is coupled (or divided). For the convenience of the description, only first core 102 and second core 104 are marked respectively with bias lines and dots, as shown in FIGS. 11A through 11C.
First waveguide 110 is formed by embedding channel-shaped first core 102 with first cladding 101 and second cladding 103, which have lower refractive indexes than the first core 102, so that signal light to be propagated is enclosed in first core 102 and is guided through first core 102.
In the same manner, second waveguide 111 is formed by embedding channel-shaped second core 104 with second cladding 103 and third cladding 105, which have lower refractive indexes than the second core 104, so that signal light to be propagated is enclosed in second core 104 and is guided through second core 104.
For example, first core 102 and second core 104 are made from GPSG, which is silica glass in the form of particles doped with dopants of germanium (Ge) and phosphorus (P), and first through third claddings 101, 103 and 105 are made from BPSG, which is silica glass in the form of particles doped with dopants of boron (B) and phosphorus (P).
As shown in FIG. 11A, first core 102 is a straight line extending parallel to a guiding direction of the guiding light. Conversely, second core 104 formed on first cladding 101 with second cladding 103 interposed as shown in FIG. 11B is parallel to first core 102 in the guiding direction (in a direction of the thickness of the optical waveguide) and is bent (curved) in a horizontal-perpendicular direction (a direction perpendicular to the guiding direction on one and the same horizontal plane; a direction of the width of the optical waveguide) so that the combination of first core 102 and second core 104 performs an optical coupling, as shown in FIG. 11A.
A part of second core 104, of which another is arranged apart from first core 102 with respect to the horizontal direction of the plane, is arranged directly above first core 102 in such a manner that the length of the portion that first core 102 and second core 104 comes closer (the optical coupling portion) is optimized as a previous design directs. As a result, power of signal light propagating through first core 102 is distributed to first core 102 and second core 104 at a rate (a ratio, a coupling rate) in accordance with the length of the optical coupling portion of first core 102 and second core 104.
As another example of an optical part formed by an optical waveguide, a multimode interference optical coupler applies to a waveguide-type multimode interference (MMI) optical coupler (a multimode interference optical divider/coupler).
The above-mentioned conventional multi-layer waveguide with PLC has second core 104 that comes partially closer to first core 102 by bending (curving) second core 104 in the horizontal direction. The distance between first core 102 and second core 104 requires being close enough for optical coupling at the optical coupling portion that second core 104 comes directly above first core 102. In other words, the thickness of second cladding 103 formed between first core 102 and second core 104 is thin enough to allow an optical coupling.
Since the thickness of second cladding 103 formed between first core 102 and second core 104 is thin, there is a possibility of unexpected optical coupling thereby resulting in cross talk when second core 104 except the optical coupling portion is relatively close to first core 102 with respect to the horizontal direction. As a solution to eliminate cross talk, second core 104 except the optical coupling portion requires being arranged distant enough from first core 102 in the horizontal direction whereupon the arrangement of claddings and cores is restricted and the object (e.g., reducing the size, highly integrating) of the multi-layer optical waveguide would not be fully attained.
In a contrary fashion to a waveguide-type optical directional coupler, a waveguide-type MMI optical coupler requires a connecting section to connect two neighboring waveguides and the connecting section should be formed at a portion where the two waveguides come closer.
Therefore, the distance of the two waveguides formed to be narrower at the portion where the connecting section is formed and to be wider at the other portion. A conventional multi-layer waveguide-type MMI optical coupler has restriction with respect to the arrangement of claddings and cores as well as the above-mentioned conventional multi-layer waveguide-type optical directional coupler.
A waveguide-type optical device, such as a waveguide-type optical switch, a waveguide-type optical deflector, or a waveguide-type optical phase controller (modulator) by utilizing physical effect exemplified by electro-optic (EO) effect, magneto-optic (MO) effect, acousto-optic (AO) effect, and thermo-optic (TO) effect is fabricated.
For example, a waveguide-type optical phase controller utilizing TO effect includes a heater installed at an optical waveguide formed on a substrate and one or more electrodes connected to the heater. Supplying electricity to the heater through the electrodes varies the temperature of the optical waveguide thereby controlling the phase of signal light propagating through a waveguide of the optical waveguide.
At that time, avoiding a part of the optical waveguide which part is not desired to vary in temperature requires a part of the cladding to part which the heater is installed to be thinner.
As a result, since the distance between the heater and the core to guide light should be thin at the portion serving as a phase controller and should be thick at the remaining portion except the phase controller portion, the arrangement of claddings and cores is restricted as well as the above-mentioned multi-layer waveguide-type optical directional coupler.